Illuminating Nonluminescent DNA Copper Nanoclusters via Protein Encapsulation: The Role of Protein Characteristics

DNA-based luminescent copper nanoclusters (DNACuNCs) have had promising applications in biosensing and bioimaging. However, a significant number of DNA sequences still form undesirable nonluminescent DNACuNCs called "dark" clusters. The scarcity of efficient and accurate approaches for turning such "dark" clusters into luminescent ones hampers their application. To overcome this problem, we have shown how protamine, a basic protein, can be used as an encapsulating agent to convert nonluminescent DNACuNCs into luminescent ones. In this method, protamine encapsulation resulted in a 2500% enhancement of the emission intensity of "dark" DNACuNCs. The results were compared with those of lysozyme and human serum albumin (HSA) as the other encapsulating agents with diverse features; however, they were found to be not as effective as protamine in illuminating the "dark" clusters. Protamine, due to its highly cationic nature and flexible conformation compared to those of lysozyme and HSA, can adjust according to the charge distribution on the surface of NCs, leading to an effective interaction supported by the binding study. It prompts the assembly of NCs into stable and well-defined three-dimensional structures with extremely small sizes of ∼1.7 nm that support the discrete electronic transitions, resulting in an exceptionally strong fluorescence emission intensity. In addition, these NCs sustained better stability over a wider pH range, making them ideal for biological applications. The approach for achieving high emission efficiency proposed here can be extended to other nonluminescent DNA-based NCs.